Electrical insulator



Dem-8, 1942- sMlTH AL 2,304,483

ELECTRICAL INSULATOR Filed July :51, 1940 3 Sheets-Sheet 1 Dec. 8, 1942.sMl'TH AL 2,304,483

ITH EEEE ER Dec. 8, 1942. o. H. SMITH ETAL. 2,304,483

ELECTRICAL INSULATOR Filed July 31, 1940 3 Sheets-Sheet '5 FIG, 7 2OINVENTORS v SMITH H. H. WHEELER ATTORNEY Patented Dec. 8, 1942ELECTRICAL INSULATOR Donald H. Smith, Hempstead, N. Y., and Herbert H.Wheeler, Millburn, N. .L, assignors to The Western Union TelegraphCompany, New York, N. Y., a corporation of New York Application July 31,1940, Serial No. 348,708

6 Claims.

This invention relates generally to electrical insulators, and moreparticularly to pole line insulators for use with telegraph, telephoneand other communication circuits.

This application is a continuatlon-in-part of our copending applicationSerial No. 243,762, filed December 3; 1938, now Patent No. 2,218,497,issued October 15, 1940.

The signaling currents employed in communication circuits are usually ofsmall amplitude, and a definite and considerable portion of each signalimpulse transmitted is required for operating the receiving apparatus,such as relays in telegraph circuits. While repeaters are employed atconvenient locations, in certain instances the repeater points arenecessarily spaced considerable distances apart and the signals reachingthe receiving apparatus may ,become considerably attenuated, and sincethe margin of operation on communication circuits under favorableconditions is relatively small, any substantial loss of signalingcurrent due to line leakage seriously interferes with the normaloperation of the system. Also, communication circuits frequently aredisposed along highways and railroad right of waysin connection withother signaling and power lines, and are subject to interferencetherefrom, and a considerable portion of the signaling current isrequired to override the transient or interfering currents set uptherein because of such adjacent signaling and power lines, and thisfurther reduces the operating margin of the communication signals.

In order to maintain the insulation of communication circuits as high aspossible, it has here- .poses. Glass and other vitreous insulators,however, are subject to various disadvantages. Pole line insulators formideal targets for malicious persons who throw stones and missiles orshoot at them, with resultant breakage of the insulators. When aninsulator is thus cracked or broken, it enables dirt and other foreignmatter carried by the elements to collect in the cracked or brokenportion and this forms a conducting leakage path over the surface of theinsulator, particularly in rainy or humid weather, and if a portion ofthe skirt or petticoat of the insulator is broken away, this reduces thelength of the leakage path over the insulator with the result that theinsulator loses its desired insulation value. Also, when insulators arecracked or broken, there is a tendency for the insulators to be pulledor to fly off the pins on which they are supported, and the wire carriedby the loose insulator may ground on the cros'sarm or swing into theadjacent wire, thus either grounding or short-circuiting thecommunication circuit or causing the same to be crossed with an adjacentcircuit. I

In anefiort to avoid breakage of insulators, it has heretofore beenproposed to employ shields, usually of metal, to protect the insulator.The installation and maintenance of such shields, however, is expensiveand is disadvantageous because the shields reduce the clearance betweenadjacent wires and also introduce additional undesired capacity betweenthe conductors of a circuit and between these conductors and ground,which capacity effect materially attenuates the signals.

A further disadvantage of glass and other vitreous insulators is thatwhen dry and clean they may provide a negligible leakage path for thesignal current, but when exposed to rain or moisture, they are subjectto considerable surface leakage, and thus the insulation of the circuitvaries between wide limits and may necessitate frequent readjustments ofthe terminal and repeater apparatus, or the insulation of the circuitmay become so low as to render the circuit inoperative. The most severeconditions of surface leakage in such insulators occur during periods ofrain, fog, and high humidity. During rainfall the outer surface of theinsulator becomes completely wet and theinner surface also becomes wetdue to splash and condensation. By reason of the relatively lowinterfacial tension characteristics of the glass with water, the splashor condensation on the inner surface of the insulator usually produces acontinuous film of water on the surface, thereby materially increasingleakage over the insulator. During periods of fog the particles ofmoisture deposited on the insulator surfaces also tend to run togetherand form a continuous film of water. During periods of high humidity andvarying temperature conditions, the moisture condenses on the surfacesof the insulator, and this also forms a continuous film of water. Eventhough the temperature conditions during periods of high humidity arenot such as to induce condensation, nevertheless considerable leakage isoften experienced as a result of the absorption of moisture from the airby the hygroscopic free aflzaliesthd other chemicals which are presentin the surface of the glass or other vitreous material.

In an efiort to prevent the formation of continuous films of water,particularly on the inner surfaces of the insulator during periods or"rain, fog and condensation, it has heretofore bee proposed to employ aspecial iridescent coating on glass insulator-s in the manner disclosedin the i fheeler and McCiinnis application, Ser. No. 621,475, filed 8,1932, now Patent No. 2,165,773, issued July 11, 1939, which coatingexhibits the characteristic of high interfacial tension with water. Suchinsulators, however, retain various inherent disadvantages of glass andother vitreous insulators, in that they are relatively fragile andsubject to breakage, andhave to be made large and bulky to provide areasonable measure of strength. Furthermore,

the material of such insulators has a relatively high coefficient ofthermal conductivity. This results in a relatively large amount ofcondensation oi moisture on both the outer and inner surfaces of theinsulators when the insulators subjected to currents of warrnermoistureb air. Since these insulators are large ant. bulky, thedielectric losses are considerable, t is dlflicult to obtain the closespacing whilh is desirable between conductors in high frequencycommunication circuits to counteract interference and crosstalk.

Various substitutes, including hard vulcanized rubber, have beensuggested in place of glass and porcelain for insulators, but none ofthese subhas proved electrically and mechanically tory in service, andnone of these insulators has the surface portion thereof that comprisesa leakage path over the insulator had the desired hig interiacialtension with water.

1 object oi this invention an insulator in which the foregoingdisadvantages of vitreous insulators and substitutes therefor areobviated.

ll. spec'rlc object is a non-breakable insulator substantially composedof a rubber compound or the like, which has improved mechanicalcharacteristics which exhibits and maintains in service high insulationcharacteristics.

another object is an insulator of the character disclosed, in which thedielectric losses are low.

A. further object is an insulator which may be mounted directly on ametal pin, without the necessity of providing a cob or shim between themetal pin and the insulator, and which will accommodate itself to pinshaving appreciably different diameters and different threads.

Still anotherobject is an insulator which exhibits the desired physicaland electrical properties, and which facilitates close spacing betweenadjacent wires of a high frequency circuit.

Other objects and attendant advantages will appear from the followingdetailed description taken in connection with the accompanying drawings,in which:

Fig. l is a view, in elevation, of a communication type insulatorsubstantially composed of a rubber compound in accordance with ourinventicn;

Fig. 2 is a. longitudinal section of Fi 1, showing the manner in whichthe insulator is mounted on its supporting pin;

Fig. 3 illustrates the relative sizes and configurations of an insulatorembodying our invention and a standard glass insulator of the typeheretofore employed for the same type of service;

Fig. 4 shows a modified form of an insulator in accordance with theinvention;

Fig. 5 illustrates the small spacing and clearance present with standardglass insulators when it is attempted to obtain the optimum spacingbetween adjacent conductors in high frequency communication circuits;

Fig. 6 illustrates the large effective clearance obtainable between theconductors of the circuit of Fig. 5, when supported by insulators inaccordance with the present invention;

Fig. 7 shows a further form of an insulator in accordance with theinvention, adapted to be mounted on a wooden cob or wooden pin;

Fig-8- shows a double petticoat type of insulator in accordance with theinvention; and

Fig. 9 is a longitudinal section of Fig. 8.

Referring particularly to Figs. 1 and 2, there is shown a communicationtype insulator substantially composed of a. vulcanized rubber compoundin accordance with the invention, the in sulator having a crown portionto, a wire groove portion ii for receiving the line wire, a reentrantskirt or petticoat portion 12, and a pin hole 13 for receiving thethreaded portion Ma of a metal insulator pin N which is secured,

a reduced portion Mb thereof, to the crossarm of a pole line in knownmanner.

The losses in signaling current occasioned oy line insulators are, ingeneral, due to three causes: First, the leakage from the lineconductor, including its tie wire, over the outer surface of theinsulator and under its skirt or petticoat to the supporting insulatorpin; second, the leakage from the line conductor and tie wire throughthe material of the insulator to the insulator pin; and third, thedielectric losses. As hereinbefore stated, the first cause of losses insignaling current, .which is surface leakage, causes considerabletrouble under rainy or humid conditions in insulators of the typesheretofore employed and in which both the outer and inner surfaces ofthe insulator may become covered with a film of moisture. The secondcause of losses will also become considerable if the material of theinsulator does nothave a high specific volumetric resistance andmaintain this high resistance in service. With respect to the thirdcause 01 losses, which are dielectric losses, these may be considerableif the material of the insulator has a high dielectric constant or ifthe mass of the insulator material disposed in the static field aboutthe line wire and the tie wire is large, these losses becomingincreasingly important in high frequency circuits since they increasewith the frequency of the communication currents employed.

In accordance with the preferred embodiment of the invention claimedherein, the insulator is composed of a soft vulcanized rubber compound,and we have discovered that if an appreciable amount of one or more ofcertain sub- .stances of the character hereinafter disclosed iscompounded with the rubber, this causes the surfaces of the rubbercompound to have a high interfacial tension with water and thus preventsthe formation of moisture films on the insulator surfaces to give theeffect obtained by the iridized coating in the aforesaid Wheeler and Mc-Ginnis patent. In other words, the moisture occurring on the surface ofthe insulator by reason of any condensation, splash or direct rain,forms in separate globules which run off the surface freely withoutleaving a trail of free moisture behind, and thus there is produced anon-Wetting surface which maintains high insulation values under themost adverse weather conditions, thereby substantially reducing any lossof signaling current dueto line breakage and obviating the necessity offrequent readjustment of the re-' peater andreceiving apparatus employedin tele- I graph and other communication circuits.

Among the substances which when compounded with rubber have been foundto produce a surface having the desired high interfacial tension withwater, are hydrocarbon waxes, including paraflln, ceresin, ozokerite,and amorphous types such as wax tailings, mineral, beeswax, and thelike, vegetable waxes such as camauba, montan, and others, and animalwaxes such as stearin, stearic acid, beeswax, Chinese insect wax,tallow, spermaceti, and the like. Also, synthetic waxes such ashydrogenated montan wax, and higher alcohols, and substances chemicallysimilar to waxes, such as lanolin, may be used. The proportions of suchsubstances may vary within relatively wide limits; satisfactory resultshave been obtained by using one to five percent by weight of various ofthe foregoing substances. These substances may be used in lesser orgreater proportions, with varying results, except that a sufficientamount of the substance must be employed to cause the surface of therubber compound to have the desired high interfacial tension with water,without employing such amount of the substance as deleteriously affectsthe physical properties of the rubber compound. The desired proportionsof these substances relative to the rubber may readily be determinedempirically in each instance. It will be apparent that various waxes,synthetic waxes and substances chemically similar to waxes, other thanthose specifically mentioned, and derivatives thereof, may be employed.In the specification and claims, wherever applicable, the term "waxlikeis employed in a generic sense to define waxes, synthetic waxes,substances chemically similar to waxes, wax substitutes and/orequivalents thereof, which exhibit high interfacial tension with waterand/ or when compounded with or incorporated in the rubber compoundcause the surface of the insulator to have the desired high interfacialtension with water.

As is well known, rubber compounds ordinarily include numerousingredients other than rubber, such as ingredients for reinforcing,filling, coloring, softening, vulcanizing, and as ageing ingredients,including the vulcanizing agents, the desired degree of softness orhardness andother physical characteristics of the rubber compound can beobtained and, in so far as obtaining the high interfacial tension withwater, the rub- -ber compound may be relatively hard when vulcanized,although, as above stated, the soft vulcanized rubber compound ispreferable.

In addition to the advantages attendant to the high interfacial tensionwithwater, it has been found that an insulator composed of a softvulcanized rubber compound in accordance with the invention isnon-porous and is substantially nonabsorbent, and even though certain ofthe materials used as fillers, pigments and other compoundingingredients may in themselves be hygroscopic, such materials aresubstantially sealed in the surface of the insulator against moisture bythe rubber and the substance or substances employed for causing thesurface to have a high interfacial tension with water. Thus theformation of conducting films resulting from the presence of freealkalies and other chemicals in glas and vitreous materials areobviated.

Furthermore, an insulator composed of a. soft vulcanized rubber compoundin accordance with the invention has a much lower coefficient of thermalconductivity than glass, hard rubber and other materials heretoforeproposed for line insulator purposes. For example, the composition ofthe'insulator herein disclosed may be such that it has a coeiilcient ofthermal conductivity of approximately .109, whereas the coeflicient ofglass varies between approximately .33 and .5, and that of hard rubberis approximately .9, these values being expressed in the English systemof measures, i. e., B. t. u. per hour per square foot per foot thicknessper degree of Fahrenheit difference in temperature. It will beappreciated that hard rubbers of specifically diflerent formulas willpresent a considerable variation in values of thermal conductivity; thisis also true with regard to the various glasses, and to a lesser extentto soft resistors, some of the well known compounding ingredientsincluding silica, clay, carbon, zinc oxide, sulphur and magnesiumcarbonate. These compounding ingredients, however, should be of suchcharacter or employed in such amounts that they will not, to a materialextent, detrimentally affect the desired physical and electricalcharacteristics of the rubber compound, particularly in regard to itsspecific electrical resistance, dielectric losses and the like. Also,various synthetic and artificial rubber and rubber-like products may beused in the compound in combination with or to replace the rubber inwhole or in part, the permissible extent of such addition or replacementbeing limited only in so far as such substitutes undesirably affect thedesired physical and electrical properties of the insulator. In thespecification and claims, wherever applicable, the term rubber compoundis employed in a generic sense to define compounds of rubber and/orsynthetic and artificial rubber, and rubber substitutes and rubber-likeproducts, including the various other compounding ingredients and thewaxlike substance or substances employed. By varyrubbers. For example,depending upon the particular formulas employed, hard rubber may have acoemcient of thermal conductivity ranging down to .093 B. t. u.; thecoeflicient of Pyrex glass may be approximately .63 B. t. u.; and softrubber .075 to .093 B. t. u.; the important consideration, however, isthat of the relative values "of soft vulcanized rubber and glass.Compounds of such rubbers having compounded therewith a waxlikesubstance or substances in accordance with the present invention willhave substantially the same coefficients of thermal conductivity asthose of the rubbers themselves. By reason of the low coefflcient ofthermal conductivity obtainable with the insulator disclosed herein, thesurface of the insulator, under conditions of varying temperature,assumes more rapidly the temperature of the surrounding air, and thisminimizes the amount of condensation of moisture. Furthermore, the skirtin the insulator of this invention may be made relatively thin, and as aresult the time required to bring the material of this portion of theinsulator to the temperature of the surrounding air is short, and hencethe amount of condensation is further reduced.

Because of the foregoing characteristics, in sulators in accordance withthe invention retain the desired insulating characteristics for aconsiderably longer period of time than vitreous and other insulatorsheretofore known. Tests have shown that insulators embodying ourinvention which have been subjected to severe conditions of rain, fogand dirt have retained their high insulating properties to a degreesubstantially in excess of that exhibited by vitreous insulators,including iridized insulators, exposed to the same conditions.

Another important advantage of the use of a soft vulcanized rubbercompound in accordance with our invention is that the insulator isnonbreakable. Also, as disclosed in Fig. 2, the pin hole [3 in theinsulator does not have to be threaded'for engagement with the threadedportion of the pin I4, but may comprise a smooth bore since the diameterof the pin hole may be made slightly less than the outer diameter of thepin, the elasticity of the rubber compound enabling the insulatorreadily to be forced on the pin, after which it resiliently grips thepin and thus minimizes or precludes the possibility of the insulatorunscrewing off the pin because of vibration or flying off the pinbecause of a pull exerted thereon by the line wire. Furthermore, theinsulator requires no wooden cob, shim or-other cushioning materialbetween the metal pin and insulator which generally has to be employedin the case of vitreous insulators to prevent cracking of the insulatorsdue to the different temperature coeflicients of expansion of theinsulator and pin. Since no cob or shim is required, the mass ofmaterial in the static field is greatly reduced, with consequentreduction of dielectric losses, and the dielectric losses present inwooden cobs are eliminated. It is, in fact, not necessary that theinsulator pin be provided with a threaded portion, since anyprotuberance or roughened surface will enable the insulator toeffectively grip the pin, and by reason of the elasticity of thematerial the insulator is adapted to receive pins with differentdiameters and different threads. If desired, and as shown in Fig. 4, theinsulator may be molded and vulcanized on the pin i5, and thus the pinand insulator become an integral structure so that there is nopossibility of the insulator flying oil the pin. An additional advantageof the insulator is that it has a dampening effect on vibration set upin the line wires, and there is no injurious or abrasive eiiect on theline wires by reason of the wires rubbing against the insulator, as isthe case when vitreous insulators are employed.

Referring again to Fig. 2, it will be seen that the crown and wiregroove portions and H are built up so that they are relativelyinflexible and thus enable the insulator to be rigidly mounted on aninsulator pin or other supporting member, and thereby preventdisplacement of the insulator on its support and accurately maintain thedesired clearance between the wire and the adjacent conductors and thesupporting crossarm, and rigidly support the line wire in apredetermined fixed position. The crown and wire groove portions,however, are sufiiciently elastic to cause them to withstand the forceof impact when struck by an object without permanent deformation of orinjury to said portions. As shown, the underlip Ila of the wire grooveis also relatively rigid and inflexible, so that it can hold the weightof the wire even when loaded with ice, at which time there may be aweight of from 300 to 400 pounds per span exerted on the pin by the linewire.

The skirt 12, however, may be provided with a very thin wall, thereby toobtain the benefits with respect to reduced condensation of moistureabove referred to, and to cause the skirt to be sufliciently flexible towithstand better and partially absorb the force of impact when struck byan object, the skirt thus materially preventing the force of impact frombeing communicated to the upper portion of the insulator and possiblycausing displacement of the insulator.

Fig. 3 illustrates the relative proportions and configurations of aninsulator in accordance with the present invention, shown in full linein the figure, and a conventional glass insulator l, the relative sizeand configuration of which is shown approximately by broken lines in thefigure. The two insulators are each designed for the same circuit or thesame class of service. In the insulator in accordance with the presentinvention, the largestdiameter of the insulator, which is across theouter edges of the reinforced lip portion Ila, is 1% inches whereas thelargest diameter of the glass insulator i, which is across the outersurfaces of the outer pettlcoat shown, is 3% inches, the necessaryoverall dimension of this insulator being approximately twice that ofthe insulator of our invention. Also, the overall height of theinsulator of the invention, from the top of the crown to the bottom ofthe skirt is 3%; inches, whereas the overall height of the glassinsulator i, from the top of its crown to the bottom of its drip pointportion, is nearly 5 inches. Furthermore, the insulator of the inventionrequires but one skirt or pettlcoat I! which may be made quite thin; askirt which at its bottom portion [2a is only inch thick has been foundsatisfactory. Since no cob or shim is required, the inner diameter ofthe skirt may be appreciably reduced, so that the skirt may closelyapproach the pin on which the insulator is mounted, as will be seen fromFig. 2, and thereby appreciably reduce the splash effect and also reducethe amount of dirt and other foreign material that may come in contactwith the inner surface of the insulator. An insulator, such as shown inFig. 3, in which the inner diameter of the bottom portion I2a of theskirt is only 1 inches has been found suitable.

In the case of high frequency circuits it is often desirable to reducethe spacing between conductors of a circuit to limit cross-talk andinterference, as shown in Figs. 5 and 6. In Fig. 5 in which standardglass insulators are used, it will be seen that, with a desired closespacing between conductors, only a very limited separation is obtainablebetween the tie wire t of conductor w and the tie wire t' of conductorw, and that the separation between the insulators is still more limited.This condition not only results in undesirable increase in capacitybetween the conductors, but also results in the possibility of seriousleakage between the conductors due to spider webs and other foreignsubstances ex-- tending or lodging between the insulators and tie wires.With the same spacing between the conductors w and w of Fig. 6, in whichinsulators of the invention are employed, it will be seen that nearlytwice the spacing is obtained between the tie wires t and t, and thatthe spacing between the insulators is many times that obtainable by thestandard glass insulators, thereby preventing undesirable increase incapacity between the conductors, and reducing the likelihood of seriousleakage between the conductors of the circuit.

If desired, and as shown in Fig. '7, the insulator maybe made so thatthe pin hole is sufficiently large to accommodate the conventionalwooden pin or cob, so that it is unnecessary to change the pins on anexisting circuit in order to replace the insulators thereonwithinsulators in accordance with the present inventhe threads of thepin 24 and prevents loosening and unscrewing of the insulator which inthe case of glass or porcelain insulators frequently occurs due tostrain and vibration. The crown and wire groove portions aresufllciently elastic to prevent deformation or injury when struck by amissile or other foreign object, although the underlip 2la of the wiregroove is relatively rigid and inflexible to hold the weight of the wireunder adverse service conditions. Notwithstanding that the insulator ofFig. 7 is used with a pin of larger diameter than the pin shown in Figs.1, 2 and 4, the spacing between the adjacent insulators is considerablygreater than that obtainable when conventional glass or porcelaininsulators are employed. While a threaded portion 24a is shown in Fig.'7, the threads may be replaced by protuberances which are suitablyspaced to enter or engage the threads on the pin. In either case, thediameter of the pin hole in the insulator is preferably made slightlyless than the diameter of the threaded longer than is possible in glassorporcelain insulators, but due to the closer spacing obtainable betweenthe petticoats (from 3 to inch, as shown) the possibility or likelihoodof rain splashing up into the space between the petticoats is minimizedor precluded, and thus a considerable portion of the leakage pathremains substantially dry even under the most ad-' quite thin, thetendency for moisture to condense or form on the surfaces of these wallsis also substantially minimized or precluded.

Since the outer wall or petticoat 32 and the I inner wall or petticoat33 are substantially cylinportion of the pin hole to cause the insulatorto resiliently grip the pin.

Figs. 8 and 9 show how the invention may advantageously be applied to adouble petticoat type of insulator. Preferably, and as shown, the outerwall or petticoat portion 32 is substantially cylindrical, withoutflaring outwardly, and thus the spacing between adjacent insulators isnot decreased even though a second petticoat portion 33 is provided: Inthe embodiment illustrated, the largest diameter of the insulator, whichis the outer diameter of the petticoat 32, is 11'; inches. The additionof the second petticoat greatly increases the effective length of theleakage path over the surface of the insulator, and what is moreimportant it more than doubles the length of the dry" leakage path, i.e., the leakage path over the under surf-ace of the insulator, while atthe same time the overall diameter of the insulator is not increased,and by reason of this fact the developed width of the leakage path isvery much less than that of the conventional double petticoat glass orporcelain type of insulator, and the area of the surface leakage path isgreatly reduced as compared to the conventional glass or porcelaininsulators heretofore employed. The various portions of the insulatorare drawn to scale-in Figs. 8 and 9.

Another advantage of the double petticoat type of rubber insulator isthat the petticoat portions 32 and 33 may be made much thinner than thepetticoat portions in glass or porcelain insulators, and thus may bespaced closely together and the slotted portion or space s between themmay be carried up much higher.

The bottom portion 32a of the outer petticoat may be only 3 inch thickor less, and the bottom portion 330 of the inner petticoat may be only-2; inch thick orv less. This not only enables the dry leakage path tobe made much drical, this simplifies the molding of the article, whichordinarily would present difficulty particularly where two petticoatsare employed; by reason of the cylindrical configuration of theinsulator it may be removed more readily from the mold and thus enableproduction to be materially speeded up and also reduce the cost ofmanufacture. It is to be understood, however, that flared petticoatportions may be employed, if desired, instead of the cylindricalconfiguration illustrated.

While there are shown and described herein certain preferred substanceand embodiments, many other and varied forms, uses and substances willsuggest themselves to those versed in the art without departing from theinvention, and the invention is therefore not limited except asindicated by the scope of the appended claims.

We claim:

1. A non-breakable pole line insulator comprising an integral bodysubstantially composed of a soft vulcanized rubber compound, saidinsulator body having crown and wire groove portions which aresufliciently massive to enable the insulator to be rigidly mounted on aninsulator supporting member and to rigidly support a line wire inpredetermined fixed position, said crown and wire groove portions beingsufllciently elastic to cause them to withstand the force of impact whenstruck by an object without permanent deformation of or injury to saidportions, said insulator body having a reentrant elastic skirt portionwhich is relatively flexible to partially absorb the force of impactwhen struck by an object and prevent substantial deformation ordisplacement of the crown and. wire groove portions, said rubbercompound including fillers, pigments, a waxlike substance and othercompounding ingredients for giving desired physical characteristics tothe compound, the character of said compounding ingredients and therelative proportions thereof being such that the compound has highspecific volumetric electrical resistance, low dielectric losses, and isnon-porous and of low hygroscopicity such that it is substantiallynon-absorbent.

2. A non-breakable pole line insulator comprising an integral bodysubstantially composed of a soft vulcanized rubber compound and adaptedto be mounted on an insulator body supporting member, said insulatorhaving crown and wire groove portions which are sufliciently massive toenable the insulator to be rigidly mounted on said insulator supportingmember and to rigidly support a line wire in predetermined fixedposition, said crown and wire groove portions being sufliciently elasticto cause them to withstand the force of impact when struck by an objectwithout permanent deformation of or injury to said portions, saidinsulator body having a reentrant elastic skirt portion which isrelatively flexible to withstand and partially absorb the force ofimpact when struck by an object, and a portion forming an inner bore forreceiving the insulator supporting member, the inner diameter 01 saidbore being less than the outer diameter of said supporting member, theportion 01 the insulator body forming the bore being suflicientlyelastic to stretch and firmly grip the supporting member in service,said rubber compound including fillers, pigments, a waxlike substanceand other compounding ingredients for giving desired physicalcharacteristics to the compound, the character of said compoundingingredients and'the relative proportions thereof being such that thecompound has high specific volumetric electrical resistance, lowdielectric losses, and is non-porous and of low hygroscopicity such thatit is substantially non-absorbent.

3. A non-breakable pole line insulator for supporting a line conductor,adapted to be mounted on a pole line insulator supporting pin and tomaintain high line insulation values over long periods of time and underadverse weather conditions, said insulator comprising an integral bodysubstantially composed of a soft vulcanized rubber compound and having apin hole for securing the same to said insulator supporting pin, a wiregroove portion for supporting the line conductor in predetermined fixedposition with respect to said supporting pin and a thin, circularreentrant skirt portion for increasing the leakage path between the lineconductor and insulator supporting pin, the inner diameter of said skirtportion being sufiiciently small to cause the skirt to closely surroundsaid supporting pin and minimize the splash effect of rain when theinsulator is in service, said rubber compound including fillers,pigments, a waxlike substance and other compounding ingredients forgiving desired physical characteristics to the compound, the characterof said compounding ingredients and the relative proportions thereofbeing such that the compound has high specific volumetric electricalresistance, low dielectric losses, and is non-porous and of lowhygroscopicity such that it is substantially non-absorbent.

4. A non-breakable pole line insulator for supporting a line conductor,adapted to be mounted on a pole line insulator supporting pin and tomaintain high line insulation values over long periods of time and underadverse weather conditions, said insulator comprising an integral bodysubstantially composed of a soft vulcanized rubber compound and having apin hole for securing the same to said insulator supporting pin, a wiregroove portion for supporting the line conductor in predetermined-fixedposition with respect to said supporting pin and a plurality of thin,depending skirt portions, one within the other, for increasing theleakage path between the line conductor and insulator supporting pin,said rubber compound including fillers, pigments, a waxlike substanceand other compounding insredienis to: giving desired physicalcharacteristics to the compound, the character of said compoundingingredients and the relative proportions thereoi being such that thecompound has high specific volumetric electrical resistance, lowdielectric losses, and is non-porous and of. low hygroscopicity suchthat it is substantially non-absorbent.

5. A non-breakable pole line insulator for supporting a line conductor,adapted to be mounted on a pole line insulator supporting pin and tomaintain high line insulation values over long periods of time and underadverse weather conditions, said insulator comprising an integral bodysubstantially composed of a soit vulcanized rubber compound and having apin hole for seeming the same to said insulator supporting pin, a wiregroove portion for supporting the line conductor in predetermined fixedpomtion with respect to said supporting pin and a thin, dependingreentrant skirt portion for increasing the leakage path between the lineconductor and insulator supporting pin, the outer surface portionof'said skirt being cylindrical in shape, said rubber compound includingfillers, pigments, a waxlike substance and other compounding ingredientsfor giving desired physical characteristics to the compound, thecharacter of said compounding ingredients and the relative proportionsthereof being such that the compound has high specific volumetricelectrical resistance, low dielectric losses, and is non-porous and oflow hygroscopicity such that it i substantially non-absorbent.

I 6. A non-breakable pol line insulator for supportin a line conductor,adapted to be mounted on a pole line insulator supporting pin and tomaintain high line insulation values over long periods of time and imderadverse weather conditions, said insulator comprising an integral bodysubstantially composed of a soft vulcanized rubber compound whichincludes rubber and fillers, pigments and other compounding ingredientsincluding a wax-like substance for giving desired physicalcharacteristics to the compound, the character of said fillers, pigmentsand other compounding ingredients including a wax-like substance and theproportions thereof relative to the rubber being such that the compoundhas a high specific electrical resistance, low dielectric losses, and isnon-porous and of low hygroscopicity such that it is'substantiallynon-absorbent, said insulator body having a pin hole for securin thesame to said insulator supporting pin, a wire groove portion forsupporting said line conductor in predetermined fixed position withrespect to said supporting member and a thin reentrant skirt portion forincreasing the leakage path between the line conductor and insulatorpin, the diameter of said pin hole being less than the diameter of saidpin to cause the insulator to resiliently grip said pin when mountedthereon.

DONALD H. SMITH.

